Ok. You take the first seashell and obviously scrape the shit off your ass, that's a given. The second seashell scrapes what little is left off your ass. The third seashell is to clean the other two off. What they don't mention is efficiency requirements and the conservation of water principal that would theoretically be in place in a overpopulated future. Ideally, if you scrape you ass with a maximum of three seashells, then they would all be easy to clean with a minimal amount of water.
What they also don't tell you is that the toilet shouldn't have water in it, but rather a biodegradable substrate such as wood chips or mulch, that would be periodically churned and covered with a layer of lime to control the smell. But that wouldn't fit the pristine image of the future that the movie was themed on.
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